The present invention relates to color change systems for spray coating apparatus, and in particular to improved color change systems for supplying conductive coating materials to electrostatic spray coating apparatus.
Color change systems for spray coating apparatus have application in industrial operations where articles are to be spray coated at a station or as they move along a production line. Where the articles are to be coated a wide variety of colors, it generally is not practical to establish separate spray stations or production lines for each color, or even to spray a long sequence of articles of one color, then another long sequence of articles of a second color, etc. Instead, it is desirable to be able to make color changes rapidly and simply at a single station.
Electrostatic spray coating devices have increased painting efficiency over nonelectrostatic types. In order to charge the paint in an electrostatic spray coating system, it is necessary to have some means of applying the charge to the paint. In some systems, charging is accomplished by an electrode, connected to a high voltage power supply and placed in close proximity to or in contact with the fluid either just prior to or very close to the point of atomization. In rotary atomization systems, the rotary atomizer is ordinarily of a conductive material and connected to the power supply, such that the atomizer itself is the electrode. Whichever type of system is used, the charging potential is usually on the order of several tens of kilovolts, and the electrostatic charging process works well when spraying nonconductive paints. However, when spraying paints which are moderately conductive, precautions must be taken to prevent the high voltage at the electrode from being short circuited to ground through the column of paint.
One prior approach was to isolate the entire paint supply and color change system from ground potential. This allowed the entire paint system to "float" at the charging potential, but had the drawback that a large amount of electrical energy was capacitively stored in the system. To prevent the capacitively stored energy from presenting a shock hazard to operating personnel, it was necessary to provide a protective enclosure around the paint system, which increased the cost of the system and required that the spraying operation be shut down and the system electrically discharged whenever necessary to replenish the supplies of paint. Also, during operation of the system, the capacitively stored energy gave rise to the possibility of a spark causing an explosion in an environment such as exists when solvent based paints are being sprayed.
Another approach was to ground the paint supplies and color changer, and to connect the spraying apparatus to the paint system through a hose that was long enough that the electrical resistance of the paint in the hose, between the high voltage at the spraying apparatus and paint system, was large enough to reduce electrical current leakage through the hose to the paint supplies to a level that did not short out the electrode or cause the charging voltage to fall to an unacceptably low level. A disadvantage of this approach was that the hose, due to its extended length, was not only very bulky and hard to manage, but it was also difficult to thoroughly flush it clean of one color of paint in preparation for spraying another color. In addition, although the extended length of the hose limited the magnitude of leakage current, leakage current nonetheless occurred and represented "wasted" charging energy.